Galaxy Formation
Galaxy formation refers to the set of physical processes through which the diffuse matter of the early universe organized itself into distinct structures— galaxies, galaxy clusters, and cosmic filaments—under the influence of gravity and interactions between gas, stars, and dark matter. It is one of the most active fields in modern cosmology.
Dark Matter Halos
Galaxy formation begins with dark matter. Density fluctuations in the primordial plasma—traces of which are visible in the cosmic microwave background—served as seeds. The slightly denser regions attracted more dark matter, gradually forming dark matter halos. These halos then captured baryonic gas, which cooled, condensed, and fragmented into stars. The first galaxies were small, irregular structures that grew by accreting gas and merging with other galaxies.
The Reionization Era
After the Dark Ages (between 380,000 years and a few hundred million years after the Big Bang), the first stars and galaxies formed. Their intense ultraviolet radiation reionized the neutral hydrogen in the intergalactic medium— this is the era of reionization. This process, whichspanned approximately 150 million to 1 billion years after the Big Bang, profoundly altered the intergalactic medium and the subsequent formation of galaxies.
Black Hole Feedback
Supermassive black holes at the centers of galaxies play a crucial role in their evolution. When they are in an active accretion phase (such as quasars), they inject enormous amounts of &energy into their host galaxy, which can halt star formation. This mechanism of feedback explains why the most massive galaxies ceased forming stars relatively early in the history of the universe.
A concrete example
The James Webb Space Telescope has revolutionized our understanding of galaxy formation by detecting massive and surprisingly well-structured galaxies just a few hundred million years after the Big Bang— much earlier and more massive than predicted by previous models. These discoveries call for a refinement of models of galaxy formation and evolution.
Frequently Asked Questions
Is the Milky Way still forming?
In a sense, yes. The Milky Way continues to form stars (about 3 solar masses per year), absorb dwarf satellite galaxies, and accumulate gas from the surrounding cosmic filaments. Galaxy formation is an ongoing process, not a past event.
Do all galaxies share the same formation history?
No. Large elliptical galaxies appear to have formed very rapidly through the merger of many progenitor galaxies, with intense star formation activity followed by a period of quiescence. Spiral galaxies like the Milky Way have a calmer, more continuous history. Dwarf galaxies have histories that vary greatly depending on their environment.
Did the first galaxies resemble today’s galaxies?
No. The first galaxies were generally smaller, more irregular, and much more active in star formation. The large, well-structured spiral galaxies we know today are the result of billions of years of evolution, mergers, and accretion. The James Webb Space Telescope now allows us to directly observe these early stages and compare primordial galaxies to present-day structures.